Semiconductor lasers are currently used in a variety of technologies and applications, including communications networks. One type of semiconductor laser is a distributed feedback (DFB) laser. A DFB laser produces a stream of coherent, monochromatic light in response to stimulated photon emission from a solid state material of the DFB laser. DFB lasers are commonly used in optical transmitters, which are responsible for modulating electrical signals into optical signals for transmission via an optical communication network.
Generally, a DFB laser includes a positively or negatively doped bottom layer or substrate, and a top layer that is oppositely doped with respect to the bottom layer. An active region, bounded by confinement regions, is included between the top and bottom layers. These structures together form the laser body. A grating is included in either the top or bottom layer to assist in producing a coherent light beam output from the DFB laser. The coherent stream of light that is produced by the DFB laser can be emitted through either longitudinal end, or facet, of the laser body. DFB lasers are typically known as single mode devices as they produce light signals at one of several distinct wavelengths. Such light signals are appropriate for use in transmitting information over great distances via an optical communications network.
One common type of DFB laser is known as a ridge waveguide (RWG) DFB laser. RWG DFB lasers are commonly fabricated so as to include a mesa structure atop the grating layer. The mesa is formed by first depositing a regrowth layer atop the grating layer, then etching away the regrowth layer to define the mesa atop a portion of the grating layer.
One challenge commonly encountered during the fabrication of RWG DFB lasers involves the etching of the regrowth layer to define the mesa. During this etching step, the grating layer itself serves as an etch stop to prevent further incursion of the etchant into the laser structure. Unfortunately, due to its periodic structure, the grating layer often fails to adequately stop the etchant as desired. Specifically, the etchant can progress past the periodic portions of the grating layer and etch into the active region of the RWG DFB laser. This can cause defects in the laser structure, which in turn can compromise the performance and/or reliability of the RWG DFB laser. These etching challenges can also be encountered in non-RWG DFB lasers.
In light of the above discussion, a need therefore exists for a DFB laser design that enables the control, or even prevention, of undesired etching of the active region and/or other portions of the DFB laser. In this way, a DFB laser structure may be produced that is well suited to offer relatively more reliable performance than some typical DFB lasers.